I expect a Hughes insider will be needed for this one. I have no idea of the design/aerodynamic compromises that go into helicopter tail designs

Given that the 500D with the T tail was about 50% heavier than the 500C with a bigger engine and 5 bladed rotor rather than a 4 bladed rotor I imagine that there are quite a few issues that are involved.

The lateral surfaces are, I would assume, of main benefit in the cruise and some dynamic pitch stability in the hover/low speed with the angled surface on the C contributing something to yaw, something that a more effective tail rotor on the -D may compensate for. The position of the top of the T may put it in a cleaner airflow than the lower mounted surface on the -C, but I expect such info is proprietary.